1. Field of the Invention
An aspect of the present invention relates to a fusing unit and an image forming apparatus, and more particularly, to a fusing unit and an image forming apparatus for reducing a material cost and improving a fixation property.
2. Description of the Related Art
As shown in FIG. 1, a conventional fusing unit includes a heating lamp 10, a heating roller 20 and a pressing roller 30. The heating lamp 10 is located inside the heating roller 20 and the heating lamp 10 generates heat. Also, the pressing roller 30 includes an elastic member (see 137 in FIG. 4) which pushes the pressing roller 30 toward the heating roller 20. A location E in which the heating roller 20 and the pressing roller 30 engage each other, is referred to as ‘a fusing nip’. A fluid developer T adhered to a printing medium P through electrifying-exposing-developing-transferring processes passes through the fusing nip E and is fused onto the printing medium P.
As illustrated in FIG. 2, the heating lamp 10 includes a tubular unit 13 made of a transparent glass wrapping a heating unit 11 that includes a tungsten filament. On an external circumference of the tubular unit 13 a reflecting film 15 is formed. The reflecting film 15 reflects heat (more precisely, infrared rays) generated by the heating unit 11. The heat is generated by an electric current applied through an electrode brush 17 onto the fusing nip E.
FIGS. 3A-3C are graphs illustrating relationships among distributions of pressure in the fusing nip E, heat flux at the fusing nip transmitted from the heating lamp 10 and the fixation property along the lengthwise direction of the heating roller 20 in the fusing unit in FIG. 1 (in a perpendicular direction with respect to the paper in FIG. 1).
As illustrated in FIG. 3A, the pressure of the fusing nip E is smaller in a center part A than in opposite end parts B in the lengthwise direction of the heating lamp 10. As illustrated in FIG. 3B, the heat flux distribution from the heating lamp 10 to the fusing nip is uniform in the lengthwise direction of the heating lamp 10 when the reflecting film 15 is uniformly formed on the tubular unit 13. As illustrated in FIG. 3C, the fixation property of the center part A is much inferior to the fixation property of the opposite end parts B.
The reference fixation property illustrated in FIG. 3C determines the quality of the fusing unit. If the fixation property at a center part A of the tubular unit 13 differs substantially from the fixation property at an end part B, the fusing unit is determined to be of poor quality. Accordingly, the reflecting film 15 is mainly formed so as to increase the heat flux of the heating lamp 10 to the fusing nip E so that the fixation property of the center part A does not differ from the fixation property at an end part B.
However, the conventional heating lamp 10 is inefficient even if the reflecting film 15 is uniformly formed on the tubular unit 13, if the pressure distribution along the lengthwise direction of the fusing nip E is not considered. Accordingly, a material cost of the apparatus rises and the fixation property deviation between the center part A and the opposite end part B is not corrected, thereby generating an inferior image.